"She wanted to be strong, athletic and fit," says Pasternak, who has also sculpted the physiques of Eva Mendes, Katherine Heigl and Vanessa Williams.

Here's how Simpson did it.

A new routine

In his fitness plan, Harley Pasternak advises working out five days per week for 25 minutes. The routine combines cardio with weight training and core exercises in five phases. This overview will help you get started.

Don't Miss

Do a 5-minute cardio warm-up on a bike, treadmill or elliptical machine. Elevating your heart rate at the beginning helps you stay in fat-burning mode for the next two stages.

2. Tip-top shape

Do one upper-body strength-training exercise like chest flies, rows or chest presses. Shoulders and biceps are worked on Day 4 and Day 5.

3. A leg up

Do one lower-body training exercise like squats or lunges.

4. Adore your core

Do an ab-strengthening exercise. Pasternak targets the mid section with various crunches that effectively work the entire core region.

5. Finish up

Do a 5-minute cardio cool down. To get lean, Simpson often intensified her workout by extending this final phase to as long as 30 minutes (keeping her total workout time to under an hour).

From strength to strength

So what makes this approach work? "We do the strength-training moves in Phases 2, 3 and 4 back-to-back without stopping," says Pasternak. Each week, that part of the workout is altered a bit to keep it challenging. As you become stronger, weights and sets increase while the repetitions per set decrease. "When the program is constantly morphing, your body is too," he adds.

What you'll need

Forget about gym memberships and weight machines -- this workout requires only a set of dumbbells, an adjustable bench and a stability ball stored at home. Simpson used an elliptical trainer and a treadmill for cardio, but walking, jogging or jumping rope are inexpensive alternatives.

The eating plan

Naturally, burning calories is only half the solution. To keep his clients properly fueled as they lose weight (Simpson followed his diet for a while), Pasternak created the "5 Factor Diet," which consists of five meals a day (breakfast, lunch, dinner and two snacks). Startling new study on obesity »

The diet focuses on lean protein (such as chicken, salmon and turkey), low-glycemic carbs (such as oatmeal and wild rice), fiber, healthy fats (like those found in olive oil) and sugar-free beverages, for a daily total of about 1,400 calories. (Check out 5factordiet.com for information on recipes and a food delivery service.)

Pasternak also suggests treating yourself to pasta, dessert or wine one day a week so you don't feel deprived.

Majorly fit movie star

"Jessica got stronger and saw increased muscular definition in her arms, legs and abs while keeping her feminine curves," says Pasternak, who has trained the star for several months. But, he adds, anyone can start seeing results in five weeks.E-mail to a friend